Welding processes that use a continuously-fed consumable electrode tend to have simple, open-loop control systems. When ready to weld, the operator depresses a trigger on the welding gun which both energizes the wire feed motor and applies welding voltage to the electrode. When the weld is completed, the operator releases the trigger, stopping the wire feed motor and opening the welding power circuit. The two subsystems, wire feed and lo welding power, are traditionally activated and deactivated at the same time.
The wire feed may be unintentionally interrupted during consumable-wire-fed arc welding due to arcing between the tube and electrode, a clogged liner, or reaching the end of the electrode. In these cases, the wire feed stalls while the welding current continues to melt the electrode. This upsets the steady-state equilibrium between the electrode melting rate and the feeding rate, and the arc length begins to increase. The arc grows until it reaches the contact tube, where it melts the tube and contaminates the weld with molten contact tube alloy, e.g. copper alloy. In the past, recovery from this event has required the replacement of the tube and time-consuming hand grinding to remove the contaminated region from the weld.
users of continuously-fed wire electrode welding devices have attempted to avoid this melting of the contact tube by simply changing their tubes often, with time intervals sufficiently short that the tube life is never exceeded, and replenishing the supply of electrode. However, premature replacement of the tubes increases costs and reduces welding productivity. Moreover, it is not always possible to know in advance when to change the tube and the electrode supply, so melting of the tip is still a common occurrence.
Commercial feed rate transducers are designed primarily for checking the average feed rates for conformance to welding procedure requirements. The transducers include averaging circuits and filters to eliminate any short-term variations in the electrical signals. These sensor systems typically have response times on the order of approximately 0.5 to 1 second. Such systems are of no value in preventing contact tube melting because the time interval between interruption of the wire feed and the onset of contact tube melting is much shorter than the response time of the sensor.
There has remained a need in the art for a way to prevent melting of contact tubes if the wire electrode feed is interrupted.